This invention generally relates to circuit subassemblies, methods of manufacture of the circuit subassemblies, and articles formed therefrom, including circuits and multi-layer circuits.
As used herein, a circuit subassembly is an article used in the manufacture of circuits and multi-layer circuits, and includes circuit laminates, packaging substrate laminates, and build-up materials, bond plies, resin coated conductive layers, and cover films. A circuit laminate is a type of circuit subassembly that has a conductive layer, e.g., copper, fixedly attached to a dielectric substrate layer. Double clad laminates have two conductive layers, one on each side of the dielectric layer. Patterning a conductive layer of a laminate, for example by etching, provides a circuit. Multilayer circuits comprise a plurality of conductive layers, at least one of which contains a conductive wiring pattern. Typically, multilayer circuits are formed by laminating one or more circuit layers together using bond plies and, in some cases, resin coated conductive layers, in proper alignment using heat and/or pressure. After lamination to form the multilayer circuit, known hole-forming and plating technologies can be used to produce useful electrical pathways between conductive layers.
Historically, circuit subassembly dielectrics have been made with glass fabric-reinforced epoxy resins. The relatively polar epoxy material bonds comparatively well to metallic surfaces such as copper foil. However, the polar groups in the epoxy resin also lead to a relatively high dielectric constant and high dissipation factor. Electronic devices that operate at higher frequencies require use of circuit dielectric material with low dielectric constants and low dissipation factors. Better electrical performance is achieved by using comparatively nonpolar resin systems, such as those based on polybutadiene, polyisoprene, or polyphenylene oxide polymer systems. An unwanted consequence of the lower polarity of these resins systems is an inherently lower bond to metallic surfaces.
In addition, as electronic devices and the features thereon become smaller, manufacture of dense circuit layouts is facilitated by use of circuit dielectric materials with a high glass transition temperature. However, when dielectric substrates with low dielectric constants, low dissipation factors, and high glass transition temperatures are used, adhesion between the conductive layer and the dielectric substrate layer can be reduced. Adhesion can be even more severely reduced when the conductive layer is a low or very low roughness copper foil (low profile copper foil). Such foils are desirably used in dense circuit designs to improve the etch definition and in high frequency applications to lower the conductor loss due to roughness.
A number of efforts have been made to improve the bonding between dielectric circuit substrates and the conductive layer surface. For example, use of various specific polymeric compositions has been disclosed. PCT Application No. 99/57949 to Holman discloses using an epoxy or phenoxy resin having a molecular weight greater than about 4,500 to improve the peel strength of a circuit laminate. U.S. Pat. No. 6,132,851 to Poutasse also discloses use of a phenolic resole resin/epoxy resin composition-coated metal foil as a means to improve adhesion to circuit substrates. U.S. Pat. No. 4,954,185 to Kohm describes a two-step process for producing a coated metal foil for printed circuit board laminates, the first being a chemical process to create a metal oxide layer on the metal substrate surface, and the second being the application of a poly(vinyl acetal)/thermosetting phenolic composition. Gardeski, in U.S. Pat. No. 5,194,307, describes an adhesive composition having one or more epoxy components and a high molecular weight polyester component. The cured adhesive layer is flexible and can be used for bonding metal foil to flexible circuit substrate (e.g., polyimide film). Yokono et al. describe improved adhesion in a copper clad circuit laminate in U.S. Pat. No. 5,569,545, obtained by use of various sulfur-containing compounds that presumably crosslink with the resin and chemically bond to the copper. The presence of sulfur-containing compounds can be undesirable, giving rise to an increased tendency to corrode. U.S. Patent Publication No. 2005/0208278 to Landi et al. discloses the use of an adhesion-promoting elastomeric layer comprising a non-sulfur curing agent. However, in practice it has been found that the elastomeric adhesion promoting layers can result in a soft surface, increasing the possibility of handling damage during processing. Finally, Poutasse and Kovacs, in U.S. Pat. No. 5,622,782, use a multi-component organosilane layer to improve foil adhesion with another substrate. Copper foil manufacturers can apply a silane treatment to their foils as the final production step, and the silane composition, which is often proprietary, is commonly selected to be compatible with the substrate of the customer.
As noted by Poutasse et al. in U.S. Pat. No. 5,629,098, adhesives that provide good adhesion to metal and substrate (as measured by peel strength) generally have less than satisfactory high temperature stability (as measured in a solder blister resistance test). Conversely, adhesives that provide good high temperature stability generally have less than satisfactory adhesion. There accordingly remains a need in the art for methods for improving the bond between a conductive metal and a circuit substrate, particularly thin, rigid, thermosetting substrates having low dielectric constants, low dissipation factors, and high glass transition temperatures, that maintains adhesiveness at high temperatures. It would be advantageous if the adhesive did not require B-staging, and/or that use of the adhesive did not adversely affect the electrical and mechanical properties of the resulting circuit materials.
Further, because circuit subassembly materials can contain synthetic organic materials with carbon and often high hydrogen contents, they can be combustible, and many applications demand that they meet flame retardancy requirements mandated by various manufacturing sectors such as the building, electrical, transportation, mining and automotive industries. To meet these stipulations, additives are used that interfere in various ways with the chemical exothermic chain of combustion
Electrical circuit subassembly compositions have typically used halogenated, particularly brominated, flame retardant additives to achieve the necessary levels of flame retardancy. In recent years, brominated flame retardants have come under scrutiny for their potential to contribute to health and environmental problems; therefore, it has become desirable to have circuit subassemblies which include flame retardant additives that are effective, yet that do not contain halogens, especially bromine and chlorine.
Alternative commonly used flame retardant additives for polymers that do not contain halogen have serious drawbacks if used in circuit subassemblies, either because of their inherent properties, or because they are less effective as flame retardants. The former drawback can lead to poor electrical properties, decreased thermal stability, and increased water absorption. The latter drawback might be overcome by use of very high loadings, but this can lead to porosity and deterioration of physical properties. These noted problems of common alternate flame retardants are exacerbated when used with a highly flammable resin system. Examples of alternative flame retardants giving these problems are flame retardant phosphorous compounds, aluminum trihydrate, borates, and the like.
Accordingly, there remains a need for non-halogen containing flame retardant thermosetting compositions that provide the desired flame retardant properties without impairing physical properties such as electrical and moisture absorption properties. It would be equally desirable to provide a rigid halogen-free circuit subassembly having good flame retardant properties as well as having improved bonding with conductive metal materials.